I think it's nice what people are saying about everyone having their own "it," defining your own "it," etc., but I also think they're missing the point of what "it" is. By definition, "it" draws other people to you, makes almost everyone either envy you or admire you or both. And many people who have "it" are not very unique, but the sort of people who follow shallow trends. Yes, a lot of people have admirable qualities that go unnoticed by others, but "it" is a particular quality and is, by definition, *very* noticed.
It's not something you define yourself. I can say this from experience. I have a bit of an obsession with trying to define "it" because I grew up with an older sister who had it, while I did/do not. Eventually I did learn to love myself despite what others thought and to become very individualistic. No amount of overt uniqueness has ever given me "it." Uniqueness is not what makes it.
"It" is an..... essence. I don't know how else to explain it. You can add up good looks, charm, self confidence, and a bunch of other appealing traits that the "it" person possesses; you can even add less appealing ones, like arrogance; but the sum is always greater than the parts. It's impossible to point at what exactly "it" is. It must be some sublime combination of different factors of appearance, personality, etc. in just the right amounts to produce a certain effect. It can never be learned. You have it, or you don't.
Here's an example. Tell me, when you look at this picture, which girl do your eyes go straight to? Who do you focus on?
Most likely, you focus on the second girl (my sister when she was around 15 years old). She has a commanding presence. You could just write it off on the fact that she's gorgeous, has a perfect smile and a killer figure. But it's more than that. You probably can't get the full experience from just one picture, but at least notice the sort of.... aura she projects. Good looks and charm are part of it, but that's not all.
See the two girls on either side of her? How they look like timid little girls in comparison? It's interesting to note that they're both wearing her clothes in the picture. It's as if they borrowed her clothes hoping some of "it" would rub off on them. My sister is the kind of girl who can make you feel inadequate and borderline insane with envy. The girl in the center of the picture (her best friend at the time) used to lie to people, saying she was my sister. They're not friends anymore. My sister figured it out when people she had never met kept expecting her to recognize them. That's what I mean by "crazy with envy," desperately wanting to be someone you're not to the point of lying about who you are.
"It" does not necessarily correlate with niceness (my sister was somewhat snobbish and disdained a lot of her admirers) or uniqueness (she followed trends, had to have the most expensive name brand shoes, called me "weird" for dressing creatively, etc.). It's an undefinable quality that some people just have.